A lithium secondary battery, which has lately been in rapidly increasing demand, comprises three basic elements, namely, a cathode, an anode, and a separator holding an electrolyte interposed between the cathode and the anode.
The cathode is made of a current collector, such as a metal foil, metal mesh and so forth, and coated with a slurry prepared by mixing an active material, an electro-conductive material, a binding agent, and a plasticizer, where necessary, in a dispersive medium.
Incidentally, for a cathode active material, a lithium-cobalt complex oxide (LiCoO2) has mainly been used.
However, disadvantages of using cobalt (Co) as the raw material of the cathode active material from the aspects of cobalt resources, and its higher price have recently become a concern, so that attention is being focused on the use of cheaper cathode materials such as lithium-manganese complex oxide (LixMn2O4) and lithium-nickel complex oxide (LiNiO2).
The lithium-manganese complex oxide (LixMn2O4) among others has been regarded as a preferable material for use as the cathode active material of the lithium secondary battery because its discharge voltage is high and the thermal stability in the charging state is relatively high.
In general, the lithium-manganese complex oxide (LixMn2O4) is synthesized by sintering a mixture of electrolytic manganese dioxide, chemical manganese dioxide, or a manganese oxide, such as Mn2O3, Mn3O4, obtained by applying heat treatment to the former, with a lithium compound (lithium carbonate, and so on) at a predetermined mixing ratio.
However, there has been pointed out a problem of the conventional lithium-manganese complex oxide (LixMn2O4) when it is used as the cathode active material of the lithium secondary battery that cycle characteristics (particularly cycle characteristics at a high temperature) are not satisfactory.
To solve such a problem, a new method of producing a lithium-manganese complex oxide has been proposed in JP-2000-281349A, which states that manganese oxide not more than 10 μm in median grain size obtained, by a process comprising the steps of applying heat treatment at a temperature in a range of 400 to 800° C. to manganese carbonate, for example, spherical in grain shape, in an atmosphere with an oxygen concentration less than 15%, and applying heat treatment again at a temperature in a range of 530 to 800° C. in an atmosphere with an oxygen concentration not less than 15%, is mixed with a lithium compound (lithium carbonate, and so on) to be subsequently sintered.
Further, in JP-2000-281349A, it is described that a lithium-manganese complex oxide with the chemical composition represented by LixMn2O4 (1.0≦x≦1.2), spherical in grain shape, and not more than 10 μm in medial grain size, having a tap density of not less than 1.8 g/cm3, can be obtained by the method proposed as above, and if the lithium-manganese complex oxide described above is used as the cathode active material of a lithium secondary battery, a lithium secondary battery having fully satisfactory cycle characteristics, even at a high temperature, can be implemented.
However, as a result of further studies conducted by the inventors, it has been found out that a lithium-manganese complex oxide sufficiently high in tap density cannot be obtained occasionally, even by the method disclosed in JP-2000-281349A, and it is therefore not possible to fully dispel apprehensions for adverse effects on the coating properties thereof against the current collector of the lithium secondary battery, and for a possibility that this might turn out to be a factor for blocking further enhancement of the performance of the lithium secondary battery.
In view of the above, it is an object of the invention to establish means for stably providing a lithium-containing complex oxide which is high in tap density and capable of exhibiting satisfactory performance as the cathode active material of a lithium secondary battery.